This is a followup to cb8d080349.
* tests/pypi.scm ("pypi->guix-package, no wheel")
("pypi->guix-package, wheels")
("pypi->guix-package, no usable requirement file.")
("pypi->guix-package, package name contains \"-\" followed by digits"):
Replace 'python-build-system' by 'pyproject-build-system'.